Plate Boundary Deformation Project

Figure 9.1:
Location of existing (red), in preparation (yellow), and pending (blue)
Mini-PBO sites in the San Francisco Bay area. Shown also (red) are currently
operating strainmeter (circles) and BARD (triangles) stations. Blue triangles
are other pending BARD stations. Black triangles are L1-system profile sites
near the Hayward fault and the UC Berkeley campus.

The Integrated Instrumentation Program for Broadband Observations of Plate
Boundary Deformation, commonly referred to as ``Mini-PBO'', is a joint project
of the BSL, the Department of Terrestrial Magnetism at Carnegie Institution of
Washington (CIW), the IGPP at UC San Diego (UCSD), and the U.S. Geological
Survey (USGS) at Menlo Park, Calif. It augments existing infrastructure in
central California to form an integrated pilot system of instrumentation for
the study of plate boundary deformation, with special emphasis on its relation
to earthquakes. This project is partially funded through the EAR NSF/IF program
with matching funds from the participating institutions and the Southern
California Integrated Geodetic Network (SCIGN).

Because the time scales for plate boundary deformation range over at least 8
orders of magnitude, from seconds to decades, no single technique is adequate.
We have initiated an integrated approach that makes use of three complementary
and mature geodetic technologies: continuous GPS, borehole tensor strainmeters,
and interferometric synthetic aperture radar (InSAR), to characterize broadband
surface deformation. Also, ultrasensitive borehole seismometers monitor
microearthquake activity related to subsurface deformation.

The project has three components. The first augments existing instrumentation
along the Hayward and San Andreas faults in the San Francisco Bay area (Figure
9.1). During July 2001 to August 2002, five boreholes were drilled
and equipped with tensor strainmeters and 3-component L22 (velocity)
seismometers (Table 9.1). The strainmeters were recently
developed by by CIW and use 3 sensing volumes placed in an annulus with 120
degree angular separation, which allows the 3-component horizontal strain
tensor to be determined. One borehole station has also been equipped with a GPS
receiver, Quanterra recording system, and downhole pore pressure sensor, and
will eventully also include a tilt sensor. The other stations are in various
stages of completion, primarily waiting for power and telemetry to be
established. The GPS antennas at these stations are mounted at the top of the
borehole casings in an experimental approach to achieve stable compact
monuments. The GPS stations complement existing Bay Area stations of
the BARD continuous network.

The 30-second GPS, and 100-Hz strainmeter and seismometer data is acquired on
Quanterra data loggers and continuously telemetered by frame relay to the BSL.
Low frequency (600 second) data (including strainmeters, for redundancy) is
telemetered using the GOES system to the USGS. All data is available to the
community through the Northern California Earthquake Data Center (NCEDC) in
SEED format, using procedures developed by the BSL and USGS to archive similar
data from 139 sites of the USGS ultra-low-frequency (UL) geophysical network,
including data from strainmeters, tiltmeters, creep meters, magnetometers, and
water well levels.

The second component of this project is to link the BARD network in central and
northern California to the SCIGN network in southern California. The
distribution of these sites allows measurement of both near-field deformation
from fault slip on the San Andreas and regional strain accumulation from
far-field stations. During Summer 2001, nine new continuous GPS sites were
installed (see Table 8.2) in the Parkfield area spanning about 25
km on either side of the San Andreas fault. One of the receivers was contributed
by the USGS and the other eight were contributed by SCIGN, while the braced
monuments for all the sites were constructed using Mini-PBO funding. The new
array augments the considerable geophysical instrumentation already deployed in
the area and contributes to the deep borehole drilling on the San Andreas fault
(SAFOD) component of Earthscope. The data are currently downloaded daily by
SCIGN and archived by SOPAC. The NCEDC is currently assuming the responsibility
for retrieving the data from these sites over their existing frame relay
circuit at Parkfield. A subset of these sites will eventually be upgraded to
real-time streaming and analyzed in instantaneous positioning mode.

The third component is InSAR, which supports skeletal operations of a 5-m
X-band SAR downlink facility in San Diego to collect and archive radar data,
and develop an online SAR database for WInSAR users. The ERS-1/2 SAR data,
which extend from 1992 until present, offer the only means for monitoring
plate boundary deformation at high spatial resolution over all of western North
America. This data set is largely unexplored mainly because data distribution
is restricted by ESA and the time consuming nature of processing phase
information. Our objective is to improve access to these data for plate
boundary research within the strict guidelines set by ESA.

Table 9.1:
Currently operating and planned stations of the
Mini-PBO network. Strainmeter installation date is given. Depth to tensor
strainmeter and 3-component seismometers in feet.

Code

Latitude

Longitude

Installed

Strainmeter

Seismometer

Location

depth (ft)

depth (ft)

OHLN

38.00742

-122.27371

2001/07/16

670.5

645.5

Ohlone Park, Hercules

SBRN

37.68562

-122.41127

2001/08/06

551.5

530.0

San Bruno Mtn. State Park, Brisbane

OXMT

37.49795

-122.42488

2002/02/06

662.7

637.3

Ox Mtn., Half Moon Bay

MHDL

37.84227

-122.49374

2002/08/06

520.6

489.2

Golden Gate Nat. Rec. Area, Sausalito

SVIN

38.03325

-122.52638

2002/08/29

527.0

500.0

St. Vincent CYO School, San Rafael

SMCB

37.83881

-122.11159

St. Mary's College, Moraga

WDCB

38.24088

-122.49628

Wildcat Mt., Sears Pt.

During this last year, the BSL and USGS installed the first Mini-PBO stations.
Boreholes were drilled by the USGS Water Resources Division crew at five sites.
The drillers used a newly purchased rig (Figure 9.2) that
experienced numerous problems (hydraulics, stuck bits, etc.), which delayed the
drilling considerably at several of the sites and significantly increased the
costs of the project.

Figure 9.3 shows the configuration of the borehole instrument
installation at the first site at Ohlone Park in Hercules (OHLN). A 6.625"
steel casing was cemented into a 10.75" hole to 625'4" depth to prevent the
upper, most unconsolidated materials from collapsing into the hole. Below this
depth a 6" uncased hole was drilled to 676'. Coring was attempted with moderate
success below 540' through poorly consolidated mudstone to about 570', and
increasingly competent sandstone below. Moderately good core was obtained from
655' to 669', so this region was selected for the strainmeter installation. The
section of the hole below about 645' was filled with a non-shrink grout into
which the strainmeter was lowered, allowing the grout to completely fill the
inner cavity of the strainmeter within the annulus formed by the sensing
volumes to ensure good coupling to the surrounding rock.

The 3-component seismometer package was then lowered to 645.5', just above the
strainmeter, on a 2" PVC pipe, and neat cement was used to fill the hole and
PVC pipe to 565'. The pipe above this depth was left open for later
installation of the pore pressure sensor in the 520-540' region. To allow water
to circulate into the pipe from the surrounding rock for the pore pressure
measurements, the the steel casing was perforated, a sand/gravel pack was
emplaced, and a PVC screen was used at this depth. The casing was then cemented
inside to 192', and outside to 16' depth. A 12" PVC conductor casing was
cemented on the outside from the surface to 16' to stabilize the hole for
drilling and to provide an environmental health seal for shallow groundwater
flow. The annulus between the 12" conductor casing and the 6.625" steel casing
was cemented from 16' to 10' depth and above was left decoupled from the upper
surface to help minimize monument instability for the GPS antenna mounted on
top of the steel casing.

The drilling procedures and hole instrument configuration were similar at the
other four sites. At San Bruno Mt (SBRN) near Brisbane, the hole was drilled to
550' with good core through competent graywacke below 520'. Reaming of the
bottom hole from the 4" core diameter to 6" was delayed considerably for
retrieval of the reaming bit that broke and got lodged in the bottom of the
hole. At Scarper's Ridge (OXMT) near Half Moon Bay, the hole was drilled to
712.7' depth with coring attempted below 653' through granite. Because core was
poorly recovered at the lowest depths due to inadequacies with the coring
system that broke up the rock, a slightly shallower depth of 660' was chosen
for the strainmeter installation.

At the Marin Headlands (MHDL) site, the drilling in October 2001 encountered
hard greenstone with some fractures and clay layers between 410-608' and red
and green chert below to 659'. Coring at around 545' was slow and poorly
recovered. A video log of the hole showed several promising strainmeter
installation regions at 500-550' depths. However, containment of high volumes
of artesianing fluids from the well became increasing problematic. The hole
was cased to 278', sand filled on the bottom, and cemented and plugged at the
top in mid-October. In August 2002, the cement and sand were rapidly drilled
out, without any artesianing problems, allowing the strainmeter and seismometer
packages to be successfully installed.

Figure 9.2:
USGS Water Resources Division rig used to drill the
Mini-PBO boreholes at the St. Vincents site.

Figure 9.3:
The Mini-PBO borehole configuration at Ohlone,
showing the emplacement of the strainmeter and seismometer instruments downhole.
The GPS receiver is mounted on the top. Figure courtesy B. Mueller (USGS).

At the St Vincents (SVIN) site near San Rafael, drilling in August 2002 also
entailed no shortage of problems. The first hole had to be abandoned after some
tungsten grinding buttons from a defective bit dislodged and could not be
retrieved from the bottom of the hole. Hammer drilling through the very hard
graywacke encountered throughout the hole also proved difficult due to the lack
of proper stabilization on the drill string. Rotary drilling, although
relatively slow, enabled penetration to 528' in the limited time available. A
video log showed a promising region devoid of open fractures near the bottom of
the hole where the strainmeter and seismometer packages were installed without
any further difficulties.

Due to the unexpectedly high costs of drilling, only 5 boreholes could be
completed under the NSF/IF grant, although additional instrumentation was
purchased in anticipation of acquiring more sites. Caltrans intends to drill
boreholes at several locations for the HFN project in the coming year that
might be suitable for Mini-PBO installations, depending on the quality of the
rock encountered at about 600' depth. Two of the already permitted potential
sites, St. Mary's College (SMCB) and Wildcat Mt. (WDCB) (Figure 9.1
and Table 9.1), would nicely complement existing
instrumentation, providing additional monitoring of the northern Hayward fault
and initiating monitoring of the southern Rodgers Creek fault north of San
Pablo bay.

The BSL is supervising GPS, power, frame relay telemetry, and Quanterra 4120
datalogger installation at all the Mini-PBO stations. Power, telemetry, and
dataloggers are currently installed at OHLN and SBRN. The frame relay circuit
at MHDL is also installed, but the power hookup has been delayed due to
permitting complications that should be resolved in Fall 2002. We are currently
establishing power and telemetry at the most recently installed MHDL and SVIN
stations. The USGS has installed solar panels at OXMT, and soon at MHDL and
SVIN, to collect the low-frequency strainmeter data prior to establishing DC
power at the sites.

The BSL is developing an experimental GPS mount for the top of the borehole
casings to create a stable, compact monument (Figure 9.4). The
antennas, using standard SCIGN adapters and domes for protection, are attached
to the top of the 6-inch metal casing, which will be mechanically isolated from
the upper few meters of the ground. The casing below this level will be
cemented fully to the surrounding rock. We have installed a GPS antenna at
OHLN (Figure 9.5). The antenna is attached to a metal pipe
symmetrically centered with respect to the casing that is welded to a cross
beam and bolted inside the top of the casing, which allows access through the
top of the casing to the 2" pipe for heat flow measurements. A similar mount
was constructed at OXMT, but it was found to have too much play in the area
where the bolts are attached to ensure long-term stability of the monument. We
are currently redesigning the mount to minimize such non-tectonic motions.
Preliminary analysis of 100 days of the GPS observations at OHLN shows that the
short-term daily repeatabilities in the horizontal components are about 0.5-1
mm. These values are similar to those obtained with more typical monuments,
such as concrete piers or braced monuments, but it is too early to assess the
long-term stability of the borehole casing monument, which might also be
affected by annual thermal expansion effects on the casing.

Figure 9.4:
Design of the Mini-PBO GPS antenna mount on top of casing.

We are in the initial stages of assessing the data quality of the Mini-PBO
instrumentation. The newly designed tensor strainmeters appear to faithfully
record strain signals over a broad frequency range. During the 9 months that
the strainmeter at OHLN has been providing high-frequency data, the strain has
been exponentially decaying (top, Figure 9.6). This large signal is
most likely due to cement hardening effects and re-equilibration of stresses in
the surrounding rock in response to the sudden appearance of the borehole.
These effects can last for many years and are the principal reason that
borehole strainmeters can not reliably measure strain at periods greater than a
few months.

Figure 9.5:
GPS antenna mounted on top of casing at OHLN. The final installation
includes a SCIGN antenna dome and a steel protective shroud that envelopes the
casing.

At periods around 1 day, tidally induced strains are the dominant strain
signal, about 3 orders of magnitude smaller than the long-term decay signal
(bottom, Figure 9.6). Since the response of the strainmeter volumes
is difficult to estimate independently, theoretically predicted Earth tides are
typically used to calibrate the strainmeters. Figure 9.7 shows the
calibrated signals in microstrain of the OHLN strainmeter over a several day
interval.

At higher frequencies, strains due to seismic events are also evident. Figure
9.8 shows a comparison of the OHLN vertical velocity
seismometer and one component of the strainmeter for an M=2 event that occurred
within 15 km of the station. Strains from this event are about an order of
magnitude smaller than the tidal strains. We are beginning to examine the
strain data for other types of transient behavior, such as episodic creep or
slow earthquake displacements.

This project is sponsored by the National Science Foundation
under the Major Research Instrumentation (MRI) program
with matching funds from the participating institutions
and the Southern California Earthquake Center (SCEC).

Under Mark Murray's supervision, André Basset, Bill Karavas, John Friday,
Dave Rapkin, Doug Neuhauser, Tom McEvilly, Wade Johnson, and Rich Clymer have
contributed to the development of the BSL component of the Mini-PBO project.
Several USGS colleagues, especially Malcolm Johnston, Bob Mueller, and Doug
Myren, played critical roles in the drilling and instrument installation
phases. Mark Murray and Barbara Romanowicz contributed to the preparation of
this chapter.